Electrode cell assemblies for the determination of ion concentrations are known in the art and said electrode cell assenblies comprise a measuring electrode, which is immersed into the solution in which the ion concentration shall be determined as well as a reference electrode, which is immersed in an electrolyte solution. The electrical conductivity between the solution in which the ion concentration shall be deternined and the electrolyte solution in which the reference electrode is immersed, is generally provided by a bridge solution, i.e., a conduit which is filled with an electrolyte, e.g., the solution into which the reference electrode is immersed, and wherein the end of said conduit is provided with a membrane so that said conduit can be immersed into the solution in which the ion concentration has to be determined, which membrane or diaphragm is an impermeable barrier which prevents the bridge solution (electrolyte solution) from flowing out. The measuring electrode has an ion sensitive member which is immersed into the solution in which the ion concentration shall be determined and said measuring electrode, furthermore, has a buffer solution filling into which the internal reference electrode of said measuring electrode is immersed. The internal reference electrode of the measuring electrode and also the reference electrode are connected with a conductor and the other ends of those conductors are connected via the recording device so that the EMF is recorded by said recording device. The measuring electrode has an ion sensitive member which is immersed into the solution wherein the ion concentration has to be determined and said measuring electrode can, e.g., be one of the well known glass electrodes in which the ion sensitive member is a glass membrane. Electrode cell assemblies in which the measuring electrode is a glass electrode are frequently used for the determination of the pH-value of the solutions.
Furthermore, it is well known in the art to determine the ion concentration in body fluids, e.g., in blood, by using an electrode cell assembly as described above and immersing the measuring electrode into a sample of the body fluid. The ion concentration of said body fluid is then recorded with the calibrated recording device. Said method, for instance, can be applied for the determination of the pH-valve of blood samples of a person by taking at certain intervals blood samples from said person and determining the pH-valve of said blood samples by using a glass electrode as the measuring electrode.
The pH-valve of the blood makes it possible to estimate the carbon dioxide concentration in the blood, i.e., if the the pH-value of the blood is sinking, this means that the carbon dioxide concentration in the blood is increasing. The increased carbon dioxide concentration in the blood shows that the organs of the living being are not provided with sufficient oxygen and this may be toxic or harmful for the organs, especially for the brain. It would be very advantageous if narcotized persons and persons who are submitted to an intensive medical treatment were continuously observed with regard to the pH-value of their blood so that by such a continuous determination of the pH-value, any increase of the CO.sub.2 -content in the organism could be noticed immediately. Specially important, however, would be such a continuous investigation if it were possible during pregnancy when any insufficient oxygen supply for the organism of the infant may occur and, furthermore, in the course of a delivery, i.e., during the period of the opening of the cervix and the extrusion of the infant. It is well known that an increase of the carbon dioxide concentration in the blood of the infant, which, e.g., might occur because of a compression of the umbilical cord during the delivery, may cause a permanent damage of the brain of the infant (cerebral palsy). Therefore, it would be very advantageous for the gynecologist if he could observe immediately any increase of the carbon dioxide concentration in the infant organism if any difficulties occur during the pregnancy and in the course of the delivery so that the gynecologist could immediately undertake the necessary steps in order to prevent any harmful influence on the brain of the infant.
One such prior art electrode cell assembly that can measure ion concentrations in living tissue, and upon which the present invention is an improvement, is described in U.S. Pat. No. 3,973,555 which is incorporated herein by reference. This electrode cell assembly has a conical or rounded tip which is in itself the ion selective member of the measuring electrode that is introduced into the tissue of a living being by penetrating the skin and being fastened thereto by an anchoring means. The anchoring means may be a spiral or helix which surrounds the conical part of the measuring electrode and pierces the skin. The electrode cell assembly basically comprises a reference electrode incorporated in a housing which is filled with an electrolyte and the housing has at least one membrane which is ion sensitive. The reference electrode is a hollow, hydrophilic tube to one end of which is fastened an ion sensitive membrane made of a Lithium-Barium-Silicate glass of low electrical resistivity. The hydrophilic glass tube is filled with a pH buffer solution of potassium chloride. The tube also has a needle-like, silver-silver chloride reference electrode located axially therein but not touching the ion sensitive membrane and glass wall and has the buffer solution surrounding same. The tube, membrane, buffer solution and electrode comprise the internal reference system. The reference electrode extends from the glass tube at the sealed end opposite the membrane and is connected to the center conductor of a coaxial cable that connects the probe to its affiliated equipment.
The hollow glass tube is fastened to the monitor electrode base unit where it extends from a hollow chamber in one end thereof. Within the chamber is another silver-silver chloride reference electrode. One remaining element is a removable tip which has an opening in one end thereof which is only slightly larger than the diameter of the glass tube of the internal reference system and through which tube the ion selective membrane end extends upon assembly of the removable tip to the probe body. A part of the electrode base is shown in FIG. 1 and the tip is shown in FIG. 2.
To utilize the pH probe all parts thereof are sterilized, the hollow chamber in the base unit is then filled with a viscous saturated potassium chloride solution utilizing a syringe and the removable tip is then assembled to the probe body. Upon assembly of the removable tip to the probe body, by displacement the viscous potassium chloride solution is forced into the annular ring formed between the glass shaft of the internal reference system and the hole in the end of the removable tip through which the glass shaft extends. This creates a thin capillary layer of saturated potassium chloride necessary for the operation of the probe. The thickness of this potassium chloride layer is less than 10 micrometers thick and has an axial length of 2 millimeters.
The finer details concerning this prior art pH probe and its operation are found in the U.S. Pat. No. 3,973,555 previously cited and incorporated herein by reference and in other literature in the field.
One of the problems in the prior art which is solved by the present invention lies in the preparation of the pH probe for use. The hollow shaft to which the ion sensitive member is affixed is made of glass and has a diameter of 1.3 millimeters. The glass shaft is relatively weak and prone to breakage, which breakage occurs too frequently when the removable reference junction is installed on the probe body for use. This breakage occurs due to the extremely small tolerance spacing of less than 20 micrometers overall between the glass shaft and the hole through the end of the removable tip. The present cost of replacing the probe when the glass shaft is broken is in the order of $700.
Another problem in the prior art is in using a syringe to charge the probe prior to use which is inconvenient and time consuming.
In accordance with one aspect of my invention, my novel disposable tip is pre-charged with an internal reference electrolyte saturated with silver chloride and including a salt having an ion common with body or tissue fluids, and a syringe is not required to charge the probe assembly with this solution.
In accordance with another aspect of my invention, I provide a tubular sleeve on my novel disposable, removable probe tip which during installation causes the tip to remain coaxially aligned with the glass shaft to prevent breakage thereof.